In general, the TFT-LCD comprises a bottom plate on which thin film transistors and pixel electrodes are formed, and a top plate on which color filters are constructed. The liquid crystal molecules are filled between the top plate and the bottom plate. During operation, a signal voltage is applied to the TFT, which is the switching element of each pixel unit. The TFT receives the signal voltage and it turns on so that data voltage carrying image information can be applied to the corresponding pixel electrode and to the liquid crystal via the TFT. When the data voltage is applied to the TFT, the orientation of the liquid crystal molecules is changed, thereby altering the optical properties and displaying the image.
FIG. 1 illustrates the typical thin film transistor structure. An active region 104 is formed by a polysilicon thin film and defined by a patterned photoresist layer (not shown in this figure) that is deposited over a glass substrate 100. Then, another patterned photoresist layer (also not shown in this figure) is formed over the substrate 100 and partially over the active region 104. Next, an ion implanting process is performed using this photoresist as a mask to form the source/drain structure 112 in the active region 104. An insulating layer 106 is formed over the active region 104 and the glass substrate 100 to serve as a gate electrode dielectric layer. A metal layer 108 is formed over the insulating layer 106. Then, a patterned photoresist layer 110 is formed over the metal layer 108 to define a gate structure 122 as shown in FIG. 2.
In FIG. 2, an additional lightly doped region adjacent to the source/drain structure 112 is formed in the active region 104 to avoid the thermionic electron effect and the punch-through phenomenon, as well as to reduce the leakage of current while the transistor is in the “off” state. An additional ion implanting process is performed to form the lightly doped region, named lightly doped drain 116, using the gate structure as the mask.
According to the prior art, at least one mask each is required to form the source/drain structure 112 and the lightly doped drain 116. If misalignment happens in an exposure process, the two lightly doped drains 116 will have different resulting widths. Even if only one lightly doped drain 116 is formed in the active region 104, such a misalignment produces a structure which shifts the electrical characteristics of the transistor. Therefore, a manufacturing method for solving the above problem is required.